The present invention relates to a pn junction diode having a pn junction, a Schottky diode having a Schottky junction, and a composite diode having both of the pn junction and Schottky junction, each of these diodes having a rectifying function.
Diodes having a rectifying function are the most fundamental semiconductor elements or components, and various types of diodes are known which have different junction structures.
FIG. 37 is a cross-sectional view showing a pn junction diode 101 having a basic planar-type pn junction. To provide the diode 101, a high-concentration n+ cathode layer 1 is formed on one of opposite surfaces of a low-concentration n drift layer 2, and a p anode region 3 is formed in a surface layer at the other surface of the n drift layer 2. Cathode electrode 4 and anode electrode 5 are formed in contact with the surfaces of the n+ cathode layer 1 and p anode region 3, respectively. The diode 101 further includes an oxide film 6 that covers the surface of the pn junction, and a protective film 7 in the form of a nitride film. A p-type peripheral region 8 is formed in a peripheral portion of the pn junction diode 101, and a peripheral electrode 11 is provided on the surface of the peripheral region 9, to extend over a part of the oxide film 6.
The n drift layer 2 is laminated by epitaxial growth on the n+ cathode layer 1 as a substrate. For example, the impurity concentrations of the n+ cathode layer 1 and n drift layer 2 are 1xc3x971019 cmxe2x88x923, and 1xc3x971015 cmxe2x88x923, respectively, and the thicknesses of these layers 1, 2 are 450 xcexcm and 10 xcexcm, respectively. The p anode region 3 is formed by implanting p-type impurities, such as boron ions, using the oxide film 6 as a mask, and thermally diffusing the implanted ions. The p anode region 3 thus formed has a surface impurity concentration of 1xc3x971019 cmxe2x88x923, and a diffusion depth of 3 xcexcm.
The graph of FIG. 38 shows a profile of the resistivity measured along a cross section of the pn junction diode 101 of FIG. 37. In FIG. 38, the vertical axis indicates the thickness as measured from the surface of the semiconductor substrate including the n cathode layer 1 and n drift layer 2, and the horizontal axis indicates the resistivity plotted on a logarithmic scale. As shown in the cross section, the diode 101 includes the p anode region 3 having a thickness of 3 xcexcm as measured from the surface of the semiconductor substrate, n drift layer 2 having a thickness of about 60 xcexcm, and the n cathode layer 1 having a low resistivity, which is formed under the n drift layer 2. Generally, the resistivity of a portion of the surface of the p anode region 3 which has the lowest resistance is about 0.01 xcexa9xc2x7m.
FIG. 39 is a cross-sectional view of a pn junction diode 102 which is a slightly modified example of the planar-type diode of FIG. 37. As in the pn junction diode 101 of FIG. 37, a high-concentration n+ cathode layer 1 and a low-concentration n drift layer 2 constitute a semiconductor substrate, and a p anode region 3 is formed in a surface layer of the n drift layer 2 of the semiconductor substrate. The pn junction diode 102 is different from the diode 101 of FIG. 37 in that a p ring region 12 having a ring-like shape and a large diffusion depth is formed at the outer periphery of the p anode region 3. While breakdown of the pn junction diode of FIG. 37 is likely to occur in the vicinity of the periphery of the p anode region 3, the p ring region 12 having a larger diffusion depth than the p anode region 3 is formed in the diode of FIG. 39, so as to decrease the gradient of the impurity concentration, thereby to prevent occurrence of the breakdown at around the p anode region 3. As a result, the breakdown occurs uniformly throughout the p anode region 3.
FIG. 40 is a cross-sectional view of a pn junction diode 103 in which p high-concentration regions 13 having a high surface impurity concentration and a large diffusion depth are formed between p anode regions 3 having a low surface impurity concentration and a small diffusion depth, as disclosed in Shimizu et al., IEEE Trans. on Electron Devices ED-31, (1984) p. 1314). When rated current is applied to the diode, the current flows through the p anode regions 3, and therefore the diode exhibits an excellent reverse recovery characteristic. In the reverse bias situation, a depletion layer spreads out from the p high-concentration regions 13, and thus the diode shows a high breakdown voltage. The p high-concentration regions 13 may also serve as the p ring region 12 as described above.
FIG. 41 is a cross-sectional view of a Schottky diode 104 having a basic Schottky junction. To form the diode 104, a Schottky electrode 15 made of a metal, such as molybdenum, which provides a high Schottky barrier, is formed on a surface of a low-concentration n drift layer 2. A cathode electrode 4 is provided on the rear surface of a n+ cathode layer 1. A p ring region 12 is formed in a surface layer of the n drift layer 2 so as to surround a contact portion of the Schottky electrode 15. With the p ring region 12 thus provided, an electric field is prevented from concentrating at the edge of the Schottky electrode 15, and the breakdown voltage of the resulting diode can be increased.
The n drift layer 2 is laminated by epitaxial growth on the high-concentration n+ cathode layer 1 serving as a substrate. For example, the n+ cathode layer 1 has a resistivity of 0.004 xcexa9xc2x7cm, and a thickness of 350 xcexcm, and the n drift layer 2 has a resistivity of 0.90 xcexa9xc2x7cm, and a thickness of 7 xcexcm.
The graph of FIG. 38 also shows a profile of the resistivity measured along a cross section of the Schottky diode 104 of FIG. 41. The vertical axis indicates the depth as measured from the surface of the semiconductor substrate comprising the n+ cathode layer and n drift layer 2, and the vertical axis indicates the resistivity plotted on a logarithmic scale. In the case of a Schottky diode having a breakdown voltage of 60 V, for example, the n drift layer 2 having a resistivity of 0.9 xcexa9xc2x7cm extends from the surface of the semiconductor substrate to a depth of about 7 xcexcm, and the n+ cathode layer 1 having a resistivity of 0.004 xcexa9xc2x7cm is formed under the n drift layer 2.
FIG. 42 is a cross-sectional view showing a Schottky diode 105 as a slightly modified example of the Schottky diode 104 of FIG. 41. In the diode 105, trenches 16 are formed in a surface layer of the n drift layer 2, and a Schottky electrode 15 made of molybdenum, for example, is formed on the surface of the n drift layer 2 and the inner walls of the trenches 16. With the trenches 16 thus provided, a contact area of the Schottky electrode 15 is increased, thereby to increase the current capacitance.
FIG. 43 is a cross-sectional view of a composite diode 106 having a pn junction and a Schottky junction. In the composite diode 106, a relatively wide p ring region 12 is formed in a surface layer of an n drift layer 2 so as to surround a contact portion of a Schottky electrode 15, such that the Schottky electrode 15 is in contact with the surface of the p ring region 12 as well as the n drift layer 2, as disclosed in Zettler, R. A. et al.: IEEE Trans. on Electron Devices ED-16, (1969) p. 58. In this case, the p ring region 12 provides a p anode region 3 of a pn junction diode. Thus, the composite diode, in which the pn junction and Schottky junction are combined, provides a low forward voltage when it is forward biased, a high breakdown voltage, and an effect of reducing noise.
FIG. 44 is a cross-sectional view of a composite diode 107 which is a modified example of the composite diode of FIG. 43. In this example, not only the p ring region 12 is formed at the periphery of the n drift layer 2 that contacts with the Schottky electrode 15, but also p anode regions 3 are formed inside the p ring region 12. The Schottky electrode 15 is formed in contact with both exposed portions of the n drift layer 2 and the surfaces of the p anode regions 3, as disclosed in Japanese Patent No. 59-35183. The exposed portions of the n drift layer 2 between the p anode regions 3 have a small width, and a depletion layer spreads out from the p anode regions 3 when a reverse bias is applied, assuring a reduced leakage current.
FIG. 45 is a cross-sectional view of a composite diode 108 which is a slightly modified example of the composite diode 107 of FIG. 44. In this example, a p anode region 3 is formed in a surface layer of the n drift layer 2, and trenches 16 having a larger depth than the p anode region 3 are formed. Further, a Schottky electrode 15 made of molybdenum, for example, is formed in contact with the surface of the p anode region 3 and the inner walls of the trenches 16. In this case, too, the provision of the trenches 15 leads to an increase in the contact area of the Schottky electrode 15, and an increase in the current capacitance.
FIG. 46 is a cross-sectional view of a composite diode 109 which is a slightly modified example of the composite diode 108 of FIG. 45. In this example, trenches 16 are formed in a surface layer of an n drift layer 2, and p anode regions 3 are formed along the inner faces of the trenches 16. Schottky electrode 15 is formed in contact with both a surface layer of the n drift layer 2 where the trenches 16 are not formed, and the surfaces of the p anode regions 3 formed along the inner walls of the trenches (see Kunori, S et al.: Proc. of 1992 Intern. Symp. on Power Semicond. Devices and ICs, Tokyo, (1992) p.69). By providing the trenches 16, and forming the p anode regions 3 on the inner walls of the trenches 16, the leakage current can be reduced in the reverse bias situation.
In the pn junction diodes of FIGS. 37, 39, 40, lifetime killers for accelerating recombination of accumulated carriers are introduced by diffusion of Au or Pt, or irradiation of electron beams, so as to increase the switching speed. However, the introduction of the lifetime killers induces or causes an increase of leakage current. Namely, the leakage current IR increases if the reverse recovery time trr is shortened by introducing a lot of lifetime killers, and the reverse recovery time trr is increased if the leakage current IR is reduced. Thus, there is a trade-off relationship between the reverse recovery time trr and the leakage current IR. As another problem, the reverse recovery waveform shows hard recovery if a lot of lifetime killers are introduced.
In the Schottky diode of FIGS. 41 and 42, there is a trade-off relationship between the ON-state voltage VF and the reverse leakage current IR. The ON-state voltage VF may be reduced by using a metal having a small barrier height, or lowering the resistance of the n region. In this case, however, the leakage current IR is undesirably increased in the reverse bias situation. If a metal having a large barrier height is used, or the resistance of the n region is increased, the leakage current IR is reduced, but the ON-state voltage VF is increased. Thus, there is a trade-off relationship between the ON-state voltage VF and the reverse leakage current IR.
The composite diodes of FIG. 43 through FIG. 46 have a parallel structure of pn junction diode and Schottky diode, and make use of advantages of the respective types of diodes. These composite diodes, however, inherit disadvantages or problems of the pn junction diode and Schottky diodes.
Furthermore, conventional diodes generally have a low ability to withstand avalanche breakdown. In particular, the pn junction has a certain radius of curvature at around a corner portion of the p anode region or p ring region, and therefore the ability to withstand avalanche breakdown is lowered due to concentration of an electric field on the corner portion, as compared with that of the planar pn junction. Thus, the conventional diodes tend to break down due to concentration of current that may result in avalanche breakdown.
It is therefore an object of the present invention to provide a diode having a high switching speed, reduced leakage current, reduced forward voltage, and high ability to withstand avalanche breakdown. It is another object to provide a method for manufacturing such a diode.
To accomplish the above object, the present invention provides a diode comprising: a first-conductivity-type cathode layer as a first region; a first-conductivity-type drift layer as a second region placed on the cathode layer and having a lower impurity concentration than the cathode layer; a generally ring-like second-conductivity-type ring region as a third region formed in a surface layer of the first-conductivity-type drift layer; a second-conductivity-type anode region as a fourth region formed in a surface layer of the first-conductivity-type drift region located inside the ring region; a cathode electrode as a first main electrode formed in contact with the second-conductivity-type cathode layer; and an anode electrode as a second main electrode formed in contact with the second-conductivity-type anode region, wherein a portion of the second-conductivity-type anode region having the lowest resistance has a resistivity which is at least {fraction (1/100)} of that of the first-conductivity-type drift layer, and the second-conductivity-type anode region has a thickness which is smaller than a diffusion depth of the second-conductivity-type ring region.
With the diode constructed as described above, the amount of minority carriers injected into the first-conductivity-type drift layer is significantly reduced, and the carriers accumulated in this layer are accordingly reduced, whereby the reverse recovery time is shortened.
Preferably, the lowest resistivity of the second-conductivity-type anode region is in a range of 0.3 to 30 times the resistivity of the first-conductivity-type drift layer. In this case, the accumulated carriers are further reduced, resulting in a further shortened reverse recovery time. Also, lifetime killers are not necessary or only a small amount of lifetime killers need to be introduced so as to control the reverse recovery time to a given value. Accordingly, the leakage current arising upon application of a reverse bias is also considerably reduced.
In the diode as described above, the second-conductivity-type anode region preferably has a diffusion depth in a range of 0.01 to 0.5 xcexcm. In this case, the total amount of impurities is reduced, which is effective to reduce the amount of minority carriers injected into the first-conductivity-type drift layer. If the thickness of the high-resistance second conductivity-type anode region is larger than 0.5 xcexcm, the forward loss in this layer is increased.
In a method for manufacturing the diode as described above, the second-conductivity-type anode region is formed by implanting second-conductivity-type ions in a dose amount of 1xc3x971010 to 1xc3x971012 cmxe2x88x922, and conducting heat treatment.
According to the manufacturing method as described above, the resistivity of the second-conductivity-type anode region can be easily controlled to be {fraction (1/100)} or higher than that of the first-conductivity-type drift layer, and its thickness can be easily controlled to 0.5 xcexcm or smaller.
If the heat treatment for forming the second-conductivity-type anode region is conducted at a temperature in a range of 300 to 600xc2x0 C., the activation rate of impurities provided by ion implantation can be suitably controlled, without significantly changing the junction structure. If the temperature of the heat treatment is less than 300xc2x0 C. or higher than 600xc2x0 C., the diffusion depth is undesirably increased.
According to another aspect of the present invention, there is provided a diode comprising: a first-conductivity-type cathode layer having a first impurity concentration; a first-conductivity-type drift layer placed on the cathode layer and having a lower impurity concentration than the cathode layer, the cathode layer and drift layer constituting a semiconductor substrate; a generally ring-like second-conductivity-type third ring region formed in a surface layer of the first-conductivity-type drift layer; a cathode electrode formed in contact with the cathode layer; a Schottky electrode as a third main electrode which contacts with a surface of the semiconductor substrate inside the second-conductivity-type ring region, so as to form a Schottky junction; and a first-conductivity-type low-concentration region as a fifth region formed in a surface layer of the drift layer located inside the second-conductivity-type ring region, the low-concentration region having a higher resistivity than the first-conductivity-type drift layer, and having a thickness that is smaller than a diffusion depth of the second-conductivity-type ring region.
In the diode as described just above, the surface of the semiconductor substrate that contacts with the Schottky electrode provides a first-conductivity-type high-resistance region having a high resistivity, which contributes to reduction of the leakage current during application of a reverse bias.
In particular, if the thickness of the first-conductivity-type high-resistance region is in a range of 0.01 to 3.0 xcexcm, more preferably, in a range of 0.01 to 0.5 xcexcm, the high-resistance region that contacts with Schottky electrode satisfactorily yields the above effect. If this thickness exceeds 0.5 xcexcm, the forward loss is increased in this layer.
Preferably, the highest resistivity of the first-conductivity-type high-resistance region is in a range of 1.2 to 12 times that of the first-conductivity-type drift layer. If the highest resistivity is less than 1.2 times the resistivity of the drift layer, the effect of reducing the leakage current is insufficient. If it exceeds 12 times, on the other hand, the forward loss in this layer is increased to a level that is not negligible.
In a method for manufacturing the diode as described above, the first-conductivity-type high-resistance region is formed by implanting second-conductivity-type ions in a dose amount in a range of 1xc3x971010 to 1xc3x971013 cmxe2x88x922, preferably, in a range of 1xc3x971010 to 1xc3x971012 cmxe2x88x922, and conducting heat treatment.
The first-conductivity-type high-resistance region may also be formed by epitaxial growth.
In the manufacturing method as described above, the resistivity of the first-conductivity-type high-resistance region can be easily controlled to be 1.2 to 12 times that of the first conductivity-type drift layer, and its diffusion depth can be easily controlled to a range of 0.1 to 0.5 xcexcm.
If the heat treatment for forming the first-conductivity-type high-resistance region is conducted at a temperature in a range of 300 to 600xc2x0 C., the activation rate of impurities provided by ion implantation can be suitably controlled, without significantly changing the junction structure. If the temperature of the heat treatment is less than 300xc2x0 C., the impurities are not sufficiently activated. If the temperature exceeds 600xc2x0 C., the diffusion depth is increased.
According to a further aspect of the present invention, there is provided a diode comprising: a first-conductivity-type cathode layer having a first impurity concentration, a first-conductivity-type drift layer placed in the cathode layer and having a second impurity concentration that is lower than the first impurity concentration; a second-conductivity-type anode region formed in a surface layer of the first-conductivity-type drift layer; a first-conductivity-type embedded region as a sixth region formed in contact with the first-conductivity-type cathode layer located below the second-conductivity-type anode region, the embedded region having a lower resistivity than the drift layer; a cathode electrode formed in contact with the cathode layer; and an anode electrode formed in contact with the anode region; wherein the first-conductivity-type embedded region is formed only inside an area defined by a vertical projection of the second-conductivity-type anode region.
In the diode constructed as described above, the thickness of the portion of the first-conductivity-type drift layer interposed between the second-conductivity-type anode region and the first-conductivity-type embedded region is reduced. When a reverse bias is applied to this diode, therefore, current that may cause avalanche breakdown flows uniformly through a relatively wide region where the embedded region is formed.
In a diode in which a generally ring-like second-conductivity-type ring region is formed in a surface layer of a first-conductivity-type drift layer, and a second conductivity-type anode region is formed in a surface layer of the first-conductivity-type drift layer located inside the second-conductivity-type ring region, a first-conductivity-type embedded region is formed only inside an area defined by a vertical projection of the second-conductivity-type ring region, and the thickness of the first-conductivity-type drift layer located on the embedded region is smaller than the thickness of the drift layer located under the ring region.
In the diode as described just above, the thickness of the portion of the first-conductivity-type drift layer that is interposed between the second-conductivity-type anode region and the first-conductivity-type embedded region is smaller than that of the drift layer under the second-conductivity-type ring region. When a reverse bias is applied to this diode, therefore, current that may cause the avalanche breakdown flows uniformly through a wide region where the first-conductivity-type embedded region is formed.
In a diode in which a generally ring-like second-conductivity-type ring region is formed in a surface layer of a first-conductivity-type drift layer, and a Schottky electrode which contacts with a surface of a semiconductor substrate inside the second-conductivity-type ring region so as to form a Schottky junction, the first-conductivity-type embedded region is formed only inside an area defined by a vertical projection of the second-conductivity-type ring region, for the same reason as described above, and the thickness of the first-conductivity-type drift layer on the first-conductivity-type embedded region is preferably smaller than that of the drift layer located under the second-conductivity-type ring region.
In one preferred form of the diode as described above, the first-conductivity-type embedded region is formed inside an area defined by a vertical projection of the second-conductivity-type ring region, with a spacing of 5 xcexcm or larger between the periphery of the embedded region and the vertical projection. With this arrangement, current which may cause avalanche breakdown flows uniformly toward the first-conductivity-type embedded region, without affecting the second-conductivity-type ring region.
In a diode including trenches formed in a surface layer of a first-conductivity-type drift layer, a first-conductivity-type embedded region formed in contact with a portion of a first-conductivity-type cathode layer located below the trenches, and a second conductivity-type anode region formed along a surface of the drift layer and inner faces of the trenches, the first-conductivity-type embedded region is formed only inside an area of a vertical projection of the outermost trenches.
In a diode including trenches formed in a surface layer of a first-conductivity-type drift layer, a first-conductivity-type embedded region formed in contact with a portion of a first-conductivity-type cathode layer located below the trenches, and a Schottky electrode which contacts with at least a part of the surface of the first-conductivity-type drift layer and inner faces of the trenches, the first-conductivity-type embedded region is formed only inside an area of a vertical projection defined by the outermost trenches.
With the diode constructed as described above, the electric field is prevented from concentrating at bottom parts of the outermost trenches, and breakdown occurs uniformly between bottom parts of inner trenches and the first-conductivity-type embedded region.